This invention is related to overvoltage, overcurrent, and overtemperature protection in switching power supplies.
The prior art techniques for overvoltage, overcurrent, and overtemperature protection are implemented using separate circuits. Each method is discussed below.
Background: Switched-Mode Power Converters
Switched-mode power converters have many applications in industry including television and monitor power supplies. The basic application is a power supply (DC output) which achieves its output regulation by means of one or more active power handling devices which are switched ON or OFF. Because the switching device, for example a switching transistor, is fully OFF or fully ON at any one time, it dissipates very little energy, and so runs coolly and efficiently. The ratio of the on-time to the off-time of the switch (duty-cycle) is varied to suit the load demand. When little energy is required, the switch dwells in the ON position for only a short time during each cycle of operation. This short on-time is used to briefly "top off" an inductive and/or capacitive reservoir which supplies the load during the switch's off period. When the load's energy demand increases, a feedback/monitoring circuit automatically increases the duty-cycle of the switch so that it remains ON for a longer period of time in each operating cycle, and increases the energy fed to the reservoir. In this way, the energy drawn from the primary power source (usually AC mains) almost exactly matches the energy demanded by the load from instant to instant, with minimal energy waste within the power supply itself.
This type of supply is distinguished from a "linear" or "dissipative" power supply, in which regulation is achieved by power handling devices whose conduction is varied continuously over a wide range, and whose state is seldom (if ever) in a full OFF or full ON condition.
Background: Overvoltage Protection Techniques
Overvoltage ("OV") protection is normally accomplished using a large Zener diode connected across the output (VOUT and GND, in FIG. 1). During an OV fault, the Zener clamps the output to a safe voltage level and in the process draws excessive current which eventually fails to a short circuit. Correct sizing of the Zener in terms of clamping level and power dissipation is critical, because there is the danger of the Zener failing (open circuit) due to an excessive power surge. A condition may also occur where the Zener sustains its clamped mode longer than necessary and dissipates enough heat to affect operation of nearby components. Additionally, the Zener may even exceed the maximum allowable temperature rating of the printed circuit board itself.
Background: Overtemperature Protection Techniques
Overtemperature ("OT") protection is obtained using a temperature-sensitive resistor which changes in value proportional to temperature, and is connected in a voltage divider circuit. The circuit is biased by a fixed reference voltage. Changes in resistance value are translated to a change in voltage output. The output is then used to drive a comparator/driver circuit which feeds a secondary-side control loop configured to turn off the supply when an overtemperature fault occurs. The additional comparator/driver circuit adds to the complexity and cost of the circuit.
Background: Overcurrent Protection Techniques
Overcurrent ("OC") protection can be implemented in either the secondary or primary circuit. In the case of flyback topology design using current-mode pulse-width modulation ("PWM"), overload protection is performed in the primary by forcing the auxiliary supply (VAUX, in FIG. 1) to the PWM control circuit (U1) to decay to its Under Voltage Lock-Out ("UVLO") voltage level by closely coupling its auxiliary winding to the main secondary output voltage (VOUT). This low cost scheme does not offer fast response during an output short circuit condition, and allows higher power dissipation to the switching devices (Q3 and CR3). Although the standard 384X series of current-mode PWM ICs have an inherent pulse-by-pulse current limit feature, this is insufficient to provide complete overcurrent protection because it does not completely shutdown converter operation.
Alternatively, OC protection can be accomplished using a current-sense resistor or current-sense transformer connected along the secondary output current path, and as part of a complex trigger circuit. Either of these circuits offer accuracy and better response than the aforementioned but are rather costly and complex.
Switched-Mode Power Converter with Triple Protection in a Single Latch
This application discloses a single circuit comprising protection against overvoltage, overcurrent, and overtemperature in an off-line switched-mode power converter. The unified protection circuit meets the need for a simple and low-cost multiple protection circuit for an off-line Switched-Mode Power Supply ("SMPS") design which uses PWM control (preferably current-mode PWM control). A single latch element (preferably a bistable device structure, such as a thyristor-connected PNP+NPN pair) provides PWM control for all three protection circuit functions. Any of the three abnormal conditions can set the latch element, and the latch element will shut down the PWM controller when it is set. Preferably the latch is reset, once set, by timing elements which combine with one of the PWM outputs.
An advantage is that the circuit is ideally suited to work with the industry standard 384X-series of current-mode PWM controllers typically used in a flyback converter. Another advantage is that the circuit is easily implemented because it allows independent triggering of the latch driver from all three abnormal conditions. Another advantage is that it uses the minimum number of low cost components while providing full protection from overvoltage, overcurrent, and overtemperature conditions. Another advantage is that the circuit reacts much faster to a short circuit condition, thereby significantly reducing the power dissipation of the switching device. Another advantage is that the reduction in thermal load translates into improved device reliability.
To implement the three basic circuit protection functions of overvoltage, overcurrent, and overtemperature in a unified circuit by conventional methods would require a minimum dual comparator IC, dual-level reference voltages, an optoisolator to couple the fault signal to the primary circuit, and a number of resistors and capacitors. The set of components adds cost, occupies more space, and decreases circuit reliability. However, in a flyback design, reduced cost can be achieved by using individual protection circuits for each protection function, and may be implemented in both the primary and secondary sides of the circuit. The problem when using such scheme is performance degradation.